The present invention relates to a lock-up control for a motor vehicle with an automatic transmission, and more particularly to a system for controlling a lock-up clutch in a torque converter fluidly disposed between an engine and an automatic transmission.
A motor vehicle is known which employs a torque converter as being disposed between an engine and an automatic transmission. The torque converter contains a hydraulic fluid as its working medium and is equipped with a lock-up clutch. This hydraulic fluid is used also in the automatic transmission. The lock-up clutch is disengaged when the vehicle operates with an accelerator pedal depressed, causing the torque converter to induce torque multiplication. When the vehicle travels at high speeds, the lock-up clutch is engaged for increased transmission efficiency of torque.
U.S. Pat. No. JP-A-62-137467 discloses a system for controlling a lock-up clutch in response to temperature of the hydraulic fluid used in the automatic transmission. When the fluid temperature is low, it is desired that the lock-up clutch is released, allowing the torque converter to effect torque multiplication. However, if, under this low temperature condition, the lock-up clutch remains released over the whole range of running modes of the vehicle, the fuel economy becomes very poor. Thus, according to this known system, a table look-up operation of a single lock-up shift point mapping is performed based on a throttle opening degree and a corrected vehicle speed. The corrected vehicle speed results from multiplying an actual vehicle speed with a correction coefficient that is variable with varying fluid temperature. Specifically, the correction coefficient assumes 1.0 when the fluid temperature is higher than a predetermined temperature value (for example 40.degree. C.), but it drops to 0.5 when the temperature is not higher than the predetermined value. Thus, under the low temperature condition, the torque converter locks up when the vehicle speed increases further beyond a normal vehicle speed value at which the torque converter normally locks up at normal temperatures higher than the predetermined temperature value. It is also suggested in U.S. Pat. No. JP-A-62-137467 to use a different lock-up shift point mapping when the temperature is not higher than 40.degree. C.
This known system fails to deal with the situation where the fluid temperature becomes excessively high since the single lock-up shift point mapping is kept used when the fluid temperature becomes higher than the predetermined temperature value of 40.degree. C.
With this known system, there is the possibility that the fluid temperature becomes excessively high during a long uphill climbing. During the uphill climbing, the vehicle operates with an accelerator pedal depressed at a third or a second gear position, so that the torque multiplication of the torque converter continues. If the component parts of the automatic transmission are exposed to this high fluid temperature for a long period of time, the frictional elements of clutches and brakes suffer from baking and thus deteriorated.
An object of the present invention is to improve a lock-up control such that when the temperature of the hydraulic fluid is about to become abnormally high, the lock-up clutch is engaged to prevent a further increase in the temperature of the hydraulic fluid.